BAY AREA RESIDENTS who love the San Francisco Giants--and quiet, peaceful commutes on Caltrain--are about to have their loyalties divided. Weeknight games at the new Pacific Bell Park will dovetail nicely with the evening commute, and conductors and passengers are grousing that the relatively tranquil mood that once pervaded rush-hour trains is going, going, gone.

On most weeknights, commute trains are filled with dazed, post-work passengers or neurotic overachievers hellbent on getting more work done on the laptop as they head up the Peninsula and into San Francisco. At times, the atmosphere approaches a college library. Some people even whisper when they talk. Of course, there are occasional loud talkers and cell-phone jockeys, but in general, it's pretty subdued. Some uninhibited souls even sleep.

But commuters and conductors got a taste of things to come last week when season ticket holders packed trains to attend a series of evening preview parties hosted by the Giants at the ballpark. There was standing room only, and some cars got so packed, would-be passengers were told to wait for the next train. Kids were screaming; beer-bellied men in Giants uniforms were high-fiving; somebody cranked up a boom box. Shell-shocked techie commuters couldn't even hear their cell phones ring.

"It's a madhouse; it's crazy; it's a nightmare," said one conductor who asked not to be named.

Things only got worse when the Giants played an honest-to-goodness exhibition game on March 31 against the Milwaukee Brewers. The trains were even more crowded. Finally, commuters were asked to get off certain trains so fans could be expressed straight to the ballpark in time for the game. Stranded on station platforms, people started flipping off trains that sped by without stopping.

Although Caltrain officials didn't plan anything special for the preview nights, they did have a contingency plan for the Giants' first home game. Extra cars, trains and conductors were added before and after the Brewers matchup to accommodate up to 4,000 additional passengers. Conductors had warned that it wouldn't be enough, and they were right.

"I suggest commuters find a single seat upstairs, invest in some earplugs and batten down the hatches on game days," advised another conductor, who also requested anonymity. "That's assuming, of course, you're lucky enough to find a seat."

There are 40 weeknight home games scheduled this season, most starting at 7:15 or 7:35pm. That means express trains leaving San Jose Diridon Station at 5 and 5:45pm will be prime targets for fans headed to the game. The 40,800-seat ballpark is effectively sold out for the season.

Caltrain has no prohibitions against eating or drinking on the trains, which is great for regular commuters during the slow ride home. But conductors fear that the lenient approach could turn game trains into a rolling party, and the cars could end up smelling like a college dorm on Saturday mornings. There was plenty of eating and drinking on board for the preview nights, although no hibachis or beer bongs were spotted.

Caltrain officials maintain that all this was just a rehearsal to figure out exactly how to accommodate baseball fans during the season. They promise it will get better. But Caltrain public information specialist Christine Dunn isn't denying that things will change on board, even if everyone has a seat.

"It is going to be a different atmosphere, but we think that's a good thing. It's going to be a fun train ride. It's going to be an experience," Dunn said. "It is public transit, so we have to let the public ride it. There are no plans for a quiet area like a dorm or something."

Commuters departing the train at 22nd Street--the last stop before the ballpark--on a recent preview night definitely had an experience, but it would be a stretch to describe it as "fun."

"That sucked," complained one rider, who was obviously not a sports fan. "Who the hell were those people? Is there a 49ers game or something?"

Easily offended train riders in Great Britain have learned to deal with the infamous soccer trains that transport fans to away games in enemy territory. Most simply keep track of when games are scheduled and figure out alternative means of transportation. It may be time for regular Caltrain riders to hotsync weeknight Giants' games into their Palm Pilots and plan on driving to work on those days. Unless, of course, they want to experience the "fun" of hell on wheels.